This invention relates to a refractory composition characterized by high abrasion resistance, and in particular, to such a composition which may be used as a refractory castable.
Refractory castables are hydraulic setting compositions. They comprise granular refractory aggregates and chemical binders. The refractory castables are shipped in dry form, and when mixed with water to the desired consistency, may be poured like concrete, tamped or rammed into place, troweled or applied with an air gun. Refractory castables take a strong hydraulic set at room temperatures and maintain good strength until the desired ceramic bond is developed as the temperature is increased. Castables are specially suited for furnace linings of irregular contours, for patching brick work and for casting special shapes which may be urgently required. Numerous castable compositions are known, with each of the known compositions, having different properties, making each one useful for different applications.
One such application involves the use of refractory castables in lining transfer lines employed in fluid catalytic cracking units used in petrochemical processes. In such units, highly abrasive catalysts travel at high speeds, thereby creating extreme erosion potential throughout the catalytic cracking unit. In such units, early abrasion resistant linings were formed from phosphate bonded refractories, which required extensive anchoring and hand ramming to install. To reduce the expense of installing phosphate bonded refractories, the refining industry began using castables with field additions of stainless steel fibers which required less anchoring on the metal shell, and which could be poured relatively quickly. Although the foregoing improved on the time and cost of installation, increased abrasion resistance was desired.
Abrasion resistant linings in petrochemical vessels are typically chemically bonded or cement bonded refractory compositions. Abrasion resistance is generally obtained by utilizing a strong, dense refractory grain such as calcined fireclay, and a strong bond consisting of aluminum orthophosphate, or calcium aluminate cement. In the case of cement, the abrasion resistant bond is achieved by using large amounts of cement, or a combination of fumed silica, cement in amounts less than ten percent, and a surface active agent which allows flow at low water contents. Improved density, which is achieved by casting at low water contents, results in a highly abrasion resistant bond at low cement levels.
Trying to effect further economies in installation, operators of the fluid catalytic cracking units started casting larger sections of transfer lines, eliminating the assembly of many smaller sections. The refractory castables used on the transfer lines were made with relatively fast setting cements, and did not stay flowable a sufficient time for use in such applications. Refractory manufacturers reformulated their abrasion resistant castables to incorporate casting grade cements to lengthen working time. These products provided the flowability and working time needed, but strengths and abrasion resistance were often lower than similar mixes containing regular calcium aluminate cement.